1. Field of the Invention
The invention relates to a magnet injector for fuel reservoir injection systems, having:
a fuel inlet and a fuel outlet; a control chamber, which communicates with the inlet; a nozzle, which communicates with the inlet; and a nozzle needle, which has a tip for closing the nozzle opening and has a shaft end that borders on the control chamber; and
a magnet valve, which has a first electromagnet, an armature, a valve chamber that communicates with the outlet via a first passage and with the control chamber via a second passage, and a throttle body which is located in the valve chamber and is connected to the armature,
wherein the throttle body, in the state of repose of the injector, is kept in a first terminal position, in which it blocks one of the two passages, and is moved toward a second terminal position, in which it opens this passage, by triggering of the first magnet. To make shorter switching times possible, the magnet valve has a second electromagnet, which upon triggering acts on the armature oppositely from the first electromagnet.
2. Description of the Prior Art
A magnet injector of this kind is already known from the book entitled xe2x80x9cDieselmotor-Management/Boschxe2x80x9d [Bosch Diesel Engine Management System], pages 274-277 (2nd Edition, 1998, published by Robert Bosch GmbH, ISBN 3-528-03873-X). At present, fuel reservoir injection systems are predominantly used in diesel engines. Along with the injectors for the cylinders, they also have a high-pressure reservoir (common rail) and a high-pressure pump for the fuel. The high-pressure pump compresses the fuel in the reservoir to the so-called system pressure, which at present can be as high as 1350 bar. This reservoir communicates with the fuel inlet of the injector.
In the known magnet injector, the magnet valve has a single electromagnet; the throttle body in its first terminal position blocks the second passage by way of which the valve chamber communicates with the control chamber, and the first passage, by way of which the valve chamber communicates with the outlet, is disposed such that it cannot be blocked by the throttle body. When the magnet is triggered, it attracts the armature, which carries the throttle body along with it until it is in its second terminal position, in which both the second passage to the control chamber and the first passage to the outlet are open.
The mode of operation of the known magnet injector, when the engine is running, can be summarized as follows.
In the state of repose, the injector is closed, and so the fuel cannot pass through the nozzle to reach the combustion chamber of the cylinder. To that end, the electromagnet of the magnet valve is not triggered, and so a valve spring keeps the throttle body in the first terminal position, in which it blocks the second passage to the control chamber. Thus the system pressure applied by the high-pressure reservoir prevails in the control chamber and also prevails in the nozzle. Since the nozzle needle borders on the control chamber with its shaft end that is opposite its tip, the pressure in the control chamber acts on the shaft end, so that a force in the direction of the tip is exerted on the nozzle needle. A nozzle spring, which serves to prestress the tip into the nozzle opening and thus to close the injector when the engine is not running and high pressure in the high-pressure reservoir is thus absent, likewise exerts a force in the direction of the tip on the nozzle needle. These two closing forces, in the state of repose, exceed the opening force also engaging the nozzle needle; this force originates in the pressure in the nozzle on the tip, which narrows at that point, of the nozzle needle.
At the onset of injection, the injector opens because the magnet valve is triggered. To that end, the so-called attracting current is carried through the electromagnet, which serves to bring about rapid opening of the magnet valve. The magnet valve then exerts a force on the armature, which exceeds the opposite force of the valve spring, so that along its motion toward the electromagnet the armature carries the throttle body along with it and puts it in its second terminal position. As a result, the second passage, by way of which the valve chamber communicates with the control chamber, is opened. Fuel can now flow out of the control chamber through this second passage into the valve chamber and can flow on out through the first passage to the fuel outlet, which communicates with the fuel tank. The pressure in the control chamber consequently drops and is rapidly lower than the pressure in the nozzle, which still is equivalent to the system pressure. Since this reduced pressure in the control chamber is now exerted on the shaft end of the nozzle needle, this closing force on the nozzle needle drops as well, and thus because of the system pressure in the nozzle the opening force predominates, and the nozzle needle is pulled out of the nozzle opening. The fuel at system pressure can now pass through the nozzle opening to emerge from the injector, and the injection begins.
The opening speed of the nozzle needle is determined by the difference between the flow from the fuel inlet into the control chamber and the flow out of the control chamber through the second passage and to the valve chamber. The shaft end of the nozzle needle penetrates into the control chamber far enough that the closing and opening forces on the nozzle needle are equalized, and it then remains in place on a cushion of fuel. This cushion is created by a fuel flow that comes to be established in the control chamber. The nozzle is now fully open, and the fuel is injected into the combustion chamber at a pressure that is approximately equal to the system pressure in the high-pressure reservoir.
At the end of the injection, the magnet valve is no longer triggered, and so the armature is forced away from the electromagnet by the force of the valve spring, and the throttle body again blocks the second passage. Consequently, as a result of the fuel continuing to flow in from the inlet, the system pressure builds up again in the control chamber. This rising pressure causes an increasing force on the nozzle needle. As soon as this closing force from the control chamber and the force of the nozzle spring exceed the opening force from the nozzle, the nozzle needle is moved toward the nozzle opening, until the nozzle opening is again closed by the tip. The closing speed of the nozzle needle is determined by the flow of fuel from the inlet into the control chamber. The injection ends when the nozzle needle reaches its bottom stop and its tip is seated in the nozzle opening. A disadvantage of this known magnet injector, however, is that its switching times are too long to enable a preinjection with replicable, small preinjection quantities of 1 mm3 and less. This is because the magnet valve used allows only a limited armature speed. The speed can be increased by increasing the attracting current, but then armature recoiling occurs to an increasing extent, which causes a ballistic mode of operation with fluctuations in quantity of up to xc2x150% of the injected quantity. Increased exhaust emissions and fluctuations in constant-velocity operation are the consequence.
It is therefore the object of the present invention to make a magnet injector of the type defined at the outset available that makes shorter switching times possible, so that even small injection quantities of less than 1 mm3 can be replicably defined.
This object is attained in that:
the magnet valve has a second electromagnet, which upon triggering acts on the armature oppositely from the first electromagnet; and
the throttle body is embodied such that in its second terminal position, it blocks the other of the two passages and along the way between its two terminal positions opens both passages.
Consequently, this magnet injector has a magnet valve with two oppositely acting electromagnets and with one common armature. In addition, the throttle body is embodied such that in one of its two terminal positions, it blocks one of the two passages that open into the valve chamber, and opens the other passage, while in its other terminal position, conversely, it opens that passage and blocks the other one.
With this magnet injector, a small injection quantity desired for the preinjection, for instance, is defined simply in that the first electromagnet is triggered with the attracting current, which then attracts the armature.
As a result, the throttle body is moved from its first terminal position to its second terminal position. The time required for this suffices to relieve the control chamber such that a small preinjection is generated. Since in both terminal positions of the throttle body, the fuel flow from the control chamber to the outlet is interrupted, but not along the stroke path of the throttle body, the preinjection is terminated without having to reverse the direction of motion of the throttle body. As a result, in comparison to the known magnet injector with only one electromagnet, the switching time can be reduced markedly.
Furthermore, because of the defined stop of the throttle body at the passage to be blocked, fluctuations in the injection quantity are avoided.
To create the main injection, both electromagnets are triggered, so that the throttle body is moved out of its second terminal position and held in a middle position, in which it opens both passages. In this middle position, the fuel flows constantly out of the control chamber through the second passage into the valve chamber and on through the first passage to the outlet and finally back to the tank. The pressure in the control chamber drops, as in the known magnet injector, so that the shaft end of the nozzle needle is pulled into the control chamber and its tip is pulled out of the nozzle opening. The fuel flowing from the inlet into the control chamber provides for the fuel cushion once the nozzle needle has reached its upper stop.
In the state of repose of the injector, the throttle body can be kept in its first terminal position as a result of the fact that the second electromagnet is triggered. In that case, while the engineering effort and expense are low, nevertheless the requisite current must be furnished by the motor, which with a view to the much longer interval between two injections, compared with the duration of the injection itself, causes a marked drop in efficiency. A valve spring is preferably provided, which prestresses the throttle body into its first terminal position.
It is also preferred that the control chamber communicates with the valve chamber via an outlet throttle and/or with the inlet via an inlet throttle. With the aid of these throttles, the flow from the fuel inlet into the control chamber and the flow out of the control chamber into the valve chamber can be predetermined as desired; these flows for instance determine the opening and closing speed of the nozzle needle or the volume of the fuel cushion in the control chamber when the injector is fully open.
It can advantageously also be provided that a compensation chamber communicates with the inlet, and that the armature is connected to an armature shaft, whose free end face borders on the compensation chamber. This is because as a result, the throttle body is almost completely forcecompensated, so that it can react quickly to the forces exerted by the electromagnet.